Waveguide Surface Roughness Research Articles

Chiral exceptional point and coherent suppression of backscattering in silicon microring with low loss Mie scatterer

Non-Hermitian systems with their spectral degeneracies known as exceptional points (EPs) have been explored for lasing, controlling light transport, and enhancing a sensor’s response. A ring resonator can be brought to an EP by controlling the coupling between its frequency degenerate clockwise and counterclockwise traveling modes. This has been typically achieved by introducing two or more nanotips into the resonator’s mode volume. While this method provides a route to study EP physics, the basic understanding of how the nanotips’ shape and size symmetry impact the system’s non-Hermicity is missing, along with additional loss from both in-plane and out-of-plane scattering. The limited resonance stability poses a challenge for leveraging EP effects for switches or modulators, which requires stable cavity resonance and fixed laser-cavity detuning. Here we use lithographically defined asymmetric and symmetric Mie scatterers, which enable subwavelength control of wave transmission and reflections without deflecting to additional radiation channels. We show that those pre-defined Mie scatterers can bring the system to an EP without post tuning, as well as enable chiral light transport within the resonator. Counterintuitively, the Mie scatterer results in enhanced quality factor measured on the transmission port, through coherently suppressing the backscattering from the waveguide surface roughness. The proposed device platform enables pre-defined chiral light propagation and backscattering-free resonances, needed for various applications such as frequency combs, solitons, sensing, and other nonlinear optical processes such as photon blockade, and regenerative oscillators.Graphical

Ultra‐Low‐Loss Silicon Nitride Photonics Based on Deposited Films Compatible with Foundries

AbstractThe fabrication processes of silicon nitride (Si3N4) photonic devices used in foundries require low temperature deposition, which typically leads to high propagation losses. Here, it is shown that propagation loss as low as 0.42 dB cm−1 can be achieved using foundry compatible processes by solely reducing waveguide surface roughness. By postprocessing the fabricated devices using rapid thermal anneal (RTA) and furnace anneal, propagation losses down to 0.28 dB cm−1 and 0.06 dB cm−1, respectively, are achieved. These low losses are comparable to the conventional devices using high temperature, high‐stress LPCVD films. The dispersion of the devices is also tuned, and it is proved that these devices can be used for linear and nonlinear applications. Low threshold parametric oscillation, broadband frequency combs, and narrow‐linewidth laser are demonstrated. This work demonstrates the feasibility of scalable photonic systems based on foundries.

Silicon Nitride Interferometers for Optical Sensing with Multi-micron Dimensions

Increasing the size of the smallest features of Photonic Integrated Circuits (PICs) to multi-micron dimensions can be advantageous to avoid expensive and complex lithographic steps in the fabrication process. In applications where extremely reduced chip size is not a requirement, the design of devices with multi-micron dimensions is potential interesting to avoid the need for e-beam lithography. Another benefit is that making the dimensions larger reduces the effect of lithographic imperfections such as waveguide surface roughness. However, the benefits do not come without limitations. Coupling the light in and out of the circuit is more challenging since diffraction gratings are not available when designing for such large dimensions. Circuit bends must have a larger radius of curvature and the existence of multimode propagation conditions can have detrimental impact in the performance of several devices, such as interferometers. In this study we perform simulations of the coupling between a lensed multimode optical fiber and a multi-micron a-SiN:H rib waveguide. Light coupling efficiency is analyzed as a function of distance variations using the FDTD method and compared with coupling to a strip waveguide. Moreover, we use numerical simulations to study the performance of a Mach-Zehnder interferometer sensitive to refractive index variations. Both the interferometer, splitters and combiners are designed with multi-micron dimensions.

Improving the photostability of printed organic photovoltaics through luminescent solar concentrators

Abstract In this study, we propose to improve the photostability of large-area (up to 19.35 cm2) printed organic photovoltaics (OPVs) through luminescent solar concentrators (LSCs), which convert high-energy short-wavelength (λ 600 nm) photons. We investigate the impacts of the luminophore concentration and waveguide surface roughness on the power conversion efficiency (PCE) of the OPV-LSCs with sizes from 1 inch (2.54 cm) to 12 inches (30.48 cm), corresponding to front surface areas from 6.4516 cm2 to 929.0304 cm2. For the 6-inch (232.26-cm2) OPV-LSCs, the results show that a high waveguide surface roughness (Rq = 63 nm) leads to devices with a high device PCE (ηLSC = 0.93%) and wide external quantum efficiency (EQE) from 400 nm to 800 nm. Detailed edge emission analysis on the luminescent waveguides indicates an edge emission with a large fraction (>95%) of long-wavelength photons comprised of a large fraction (>25%) of near-infrared (NIR) (λ > 700 nm) light. This leads to the photostability (defined as the hours when the PCE drops by 10%) of the OPV-LSCs significantly improved to 320 h under continuous 1 sun illumination (the photostability of the OPVs is only 25 h). The approach of OPV-LSCs paves the way to practically improve the photostability of large-area printed OPVs.

High-Q suspended optical resonators in 3C silicon carbide obtained by thermal annealing.

We fabricate suspended single-mode optical waveguides and ring resonators in 3C silicon carbide (SiC) that operate at telecommunication wavelength, and leverage post-fabrication thermal annealing to minimize optical propagation losses. Annealed optical resonators yield quality factors of over 41,000, which corresponds to a propagation loss of 7 dB/cm, and is a significant improvement over the 24 dB/cm in the case of the non-annealed chip. This improvement is attributed to the enhancement of SiC crystallinity and a significant reduction of waveguide surface roughness, from 2.4 nm to below 1.7 nm. The latter is attributed to surface layer oxide growth during the annealing step. We confirm that the thermo-optic coefficient, an important parameter governing high-power and temperature-dependent performance of SiC, does not vary with annealing and is comparable to that of bulk SiC. Our annealing-based approach, which is especially suitable for suspended structures, offers a straightforward way to realize high-performance 3C-SiC integrated circuits.

Effect of surface roughness of optical waveguide on imaging quality and a formula of RSE tolerance and incident angle.

The optical waveguide is a lightweight and portable scheme for augmented reality near-eye display devices. However, the surface roughness of the waveguide affects its imaging performance, which has not been studied. In this work, we investigate the light scattering caused by the root-mean-square roughness of the waveguide surface and present two methods to numerically analyze the modulation transfer function (MTF) of the display system. Here, we consider the effects of different surface roughness, incident angle, and incident wavelength on the scattering distribution when other conditions are constant. For a simplified optical waveguide display system, the MTF degradation and the variation of the tolerance is calculated. And when the MTF (@ 40 cycles/mm) is required to be 0.3 and the incident angles of the total reflection surface are 45°, 55°, 65° and 75°, the random surface error (RSE) tolerances are 0.207λ0, 0.255λ0, 0.347λ0 and 0.566λ0 (λ0=0.5461µm), respectively. We find a formula descripting the relationship between RSE tolerance and incident angle. If the RSE tolerance exceeds the value of the formula at an angle, the imaging quality of the system will drop significantly. The formula can predict tolerances and incident angles and provide basic tool for imaging quality analysis and manufacturing for optical waveguide AR/VR display systems.

Effective metrology and standard of the surface roughness of micro/nanoscale waveguides with confocal laser scanning microscopy.

Surface roughness is one of the important parameters affecting the optical scattering loss of dielectric waveguides. Despite extensive research interests in correlating optical losses with surface roughness, not much research focus has been dedicated to the study of an accurate metrology and measurement standard for the characterization of waveguide roughness. In this Letter, we report an effective metrology for the measurement of waveguide surface roughness, using confocal laser scanning microscopy (CLSM). We also provide the definition of surface roughness relevant to CLSM terminology based on the measured peak-to-valley (P-V) values, which can be correlated to the conventional root-mean-square roughness, by employing multi-dimensional statistical models. Finally, we demonstrate the use of CLSM metrology in measuring two-dimensional roughness of 10 μm×6 μm silica waveguides, showing an average top surface roughness of 0.151μm and an average sidewall roughness of 0.203μm. For comparison, the scanning electron microscopy (SEM) measurements are also carried out for the same waveguide samples, and then the measured sidewall roughness values are in the range of 0.08-0.12μm. Since SEM measures only the amplitude of roughness profile, while CLSM measures the P-V value, after doubling the SEM value, these two methods can provide comparable results of roughness.

Fabrication and optical characterization of GaN waveguides on (−201)-oriented β-Ga_2O_3

Gallium nitride (GaN), a wide-bandgap III–V semiconductor material with a bandgap wavelength λg = 366 nm (for Wurtzite GaN) and transparency window covering the visible spectrum, has a large number of applications for photonics and optoelectronics. However, the optical quality of this material suffers from growth imperfections due to the lack of a suitable substrate. Recent studies have shown that GaN grown on (−201) β – Ga2O3 (gallium oxide) has better lattice matching and hence superior optical quality as compared to GaN grown traditionally on Al2O3 (sapphire). In this work, we report on the fabrication of GaN waveguides on Ga2O3 substrate, followed by a wet-etch process aimed at the reduction of waveguide surface roughness and improvement of side-wall verticality in these waveguides. The propagation loss in the resulting waveguides has been experimentally determined to be 7.5 dB/cm.

Linear and nonlinear characterization of low-stress high-confinement silicon-rich nitride waveguides.

In this paper we introduce a low-stress silicon enriched nitride platform that has potential for nonlinear and highly integrated optics. The manufacturing process of this platform is CMOS compatible and the increased silicon content allows tensile stress reduction and crack free layer growth of 700 nm. Additional benefits of the silicon enriched nitride is a measured nonlinear Kerr coefficient n(2) of 1.4·10(-18) m(2)/W (5 times higher than stoichiometric silicon nitride) and a refractive index of 2.1 at 1550 nm that enables high optical field confinement allowing high intensity nonlinear optics and light guidance even with small bending radii. We analyze the waveguide loss (∼1 dB/cm) in a spectrally resolved fashion and include scattering loss simulations based on waveguide surface roughness measurements. Detailed simulations show the possibility for fine dispersion and nonlinear engineering. In nonlinear experiments we present continuous-wave wavelength conversion and demonstrate that the material does not show nonlinear absorption effects. Finally, we demonstrate microfabrication of resonators with high Q-factors (∼10(5)).

Investigation into defects occurring on the polymer surface during the photolithography process

With a view to improving the realisation of polymer optical waveguide some features relevant to the photolithography process are analysed. This paper focuses on defects that occur on the surface of polymer layers involved in the process. For example, depending on the heat treatment or the deposited material, some worm-like defects appear on the polymer surface. When they occur, the waveguide surface roughness becomes too high (about one hundred nm and more). This means that the optical performance of the waveguides is too poor. In this document, we show the changes in temperature on polymer film surfaces which are coated with a thin inorganic layer and the occurrence of these defects is observed. This work confirms that the defect occurrence is clearly linked to the glass transition temperature. The paper reports that, in some cases, the adjustment of thermal properties by annealing can advantageously shift the glass transition, without changing the target optical properties.

Fabrication and integration of VLSI micro/nano-photonic circuit board

We report on the design, fabrication and integration of a modular system that we newly propose as “optical printed circuit board (O-PCB).” The O-PCB is designed to provide micro/nano-scale optical waveguides to perform the functions of transporting, switching, routing and distributing optical signals on flat modular boards or substrates that can lead to a platform for VLSI micro/nano-photonic integration. We fabricated and assembled O-PCBs using polymer-based optical waveguides, which we fabricated by thermal or ultra-violet (UV) imprinting using silicon masters. For nano-scale VLSI photonic integrated circuit applications, we used photonic crystal devices and plasmonic devices. We present the use of near-field scanning optical microscopy (NSOM) for optical characterization of micro/nano-scale photonic devices and also examine the effect of the waveguide surface roughness on the transmission characteristics of the waveguides. We present the results of calculation and simulation study that we pursue in search of scaling rules for the miniaturization and integration of waveguide devices, which we use as a guide for our design of VLSI waveguide circuits.

Fabrication of Ridge Waveguides by UV Embossing and Stamping of Sol-Gel Hybrid Materials

Multimode large core (45×35 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) optical ridge waveguides are fabricated in the room temperature by ultraviolet (UV) imprinting of soft rubber template with the organic-inorganic sol-gel hybrid materials. The fabrication processes may be either well-known UV embossing or stamping. A new sol-gel hybrid material that is thermally stable has been developed for the fabrication of low-loss optical waveguides. The UV curable methacrylic sol-gel hybrid material is used for the core layer and fluorinated methacrylic sol-gel hybrid material is used for the cladding layer. The optical propagation loss of fabricated waveguide measured by the cutback method is 0.36 dB/cm at 850 nm and the surface roughness of waveguide is about 5 /spl Aring/ root mean square.

In-plane scattering in planar optical waveguides: refractive-index fluctuations and surface roughness

The problem of the in-plane scattering of a TE guided wave by both waveguidesurface roughness and refractive-index fluctuations has been analyzed. Simple formulas emerge for the relative strength of the two scattering mechanisms and for the power collected by an off-axis detector. The theory is illustrated by applying it to the case of a step-index slab waveguide.

Laboratory measurements of mode interaction due to waveguide surface roughness

Real ocean waveguides may be strongly influenced by a rough surface bounding the duct. In order to study this scattering problem in its most fundamental form, a rigid-walled, two-dimensional guide with one surface smooth and the other surface either smooth, corrugated, or randomly rough, has been set up in the laboratory. The waveguide is hundreds of depths in extent, is driven by either a “point” source or a mode generating source, and uses either pulse or continuous wave signals. The redistribution of energy among waveguide modes, as a function of the rough surface parameters, is discussed. [Research supported by ONR.]

Scattering of optical guided waves by waveguide surface roughness: a three-dimensional treatment

The scattering of optical guided waves by waveguide surface roughness has been analyzed in three dimensions. The scattered fields were determined by satisfying the boundary conditions to first order in the height of the surface roughness. The utility of the theory for practical problems is illustrated by developing explicit formulas for the power radiated by a TE guided wave into the medium above the planar waveguide. These formulas are expressed in terms of the surface autocorrelation function.